1. Technical Field
The present invention relates to pivot rods, such as push rods of the type found included in fuel injector valve drive trains and engine cylinder valve drive trains. In particular, the invention is directed to such pivot rods and methods for their manufacture wherein a pivot element formed of a ceramic material is joined to a nonceramic rod as a tip portion thereof.
2. Background Art
In copending, commonly owned U.S. patent application Ser. No. 022,229, filed Mar. 5, 1987, now U.S. Pat. No. 4,794,894, a pivot rod, such as a push tube of the type used in engine drive trains for operating fuel injectors and cylinder valves, has a ceramic pivot rod insert attached to an end of a mounting shaft by a direct interference fit securement without exceeding the maximum tensile principle stress of the ceramic material, either during assembly or during use, despite the fact that the insert projects axially beyond the end of the mounting shaft and despite manufacturing tolerances of the mounting shaft and pivot insert. As a result, a dramatically increased wear life, as is associated with the use of wear resisting ceramic materials, is obtained without requiring uneconomical precision tolerancing of the parts as a means for dealing with the fracture problem associated with the low tensile strength of such ceramic materials.
In the embodiments of said commonly owned application, a ceramic pivot element is secured within a receiving space of a mounting shaft which may be formed by either the hollow interior of a piece of tubular stock material or may be a recess machined into an end of a piece of solid rod stock material. In the first case, and in one form of the second case, a butt joint is formed between a circumferential shoulder of the ceramic pivot element and an end of the metal shaft. In such a case, in use, loads are transmitted from the pivot element to the shaft at this abutment joint, which can result in brittle cracking and failure of the ceramic, should the shoulder be loaded beyond the normal design load.
However, in the second case, such a problem can be avoided by producing an abutment joint, not between a circumferential shoulder of the ceramic pivot element and an end of the shaft, but rather between the bottom end of the ceramic pivot element and a bottom wall of the recess forming the receiving space of the shaft within the solid rod body. Nevertheless, since the joining of the ceramic pivot element to the mounting shaft is produced via an interference fit securement that produces plastic deformation of the peripheral wall of the mounting shaft at the portion defining the receiving space, a stress concentration occurs at the junction of the peripheral and bottom walls of the recess, where the peripheral wall is restrained from radial expansion. Thus, despite the ductility of the metal shaft, it is possible for excessive stress concentrations to be produced to an extent that may lead to failure of the metal shaft or joint at that location.
In Japanese Patent Application No. 57-13203, a push rod is disclosed wherein a ceramic tip element is indirectly joined to a solid plastic (carbon reinforced resin) shaft, with an end of the ceramic element abutting an end of the shaft, via a joint formed by a separate metallic pipe. The metallic pipe joint receives an end of the rod and is secured thereto by a friction fit, while the metallic pipe is joined to the ceramic tip element by being deformed radially into a circumferential indentation provided on the periphery of the ceramic tip element. This metallic pipe is formed of a material having a coefficient of a thermal expansion between that of the ceramic tip element and the carbon fiber reinforced resin shaft to prevent breakage of the ceramic tip element. However, the disclosure of this Japanese application does not deal with the basic problem of trying to create an interference fit between a metallic pipe element and a ceramic pivot element, and possesses the shortcoming that the higher coefficient of thermal expansion of the metallic pipe (combined with the effect of the even high coefficient of thermal expansion of the resin shaft) relative to that of the ceramic tip element which it joins to the resin shaft can result in the tip becoming loose at elevated temperatures, or as a result of repeated heating and cooling of the push rod, to an extent that undesirable behavior characteristics will be achieved, particularly at increased operational speeds.
Thus, it is desirable to enable the benefits of the above-noted, commonly owned invention to be achieved without creation of undesirably high stress concentrations and without creating a pivot rod that is subject to overstressing, without going to a sleevetype attachment that is subject to the shortcomings of an arrangement as disclosed in the noted Japanese Application.